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Figure 3.41 shows an example of large-scale lateral spreading caused by liquefactionduring the Loma Prieta earthquake on October 17, 1989. As shown in Fig. 3.41, as the dis-placed ground breaks up internally, it causes fissures, scarps, and depressions to form atground surface. Notice in Fig. 3.41 that the main ground surface cracks tend to develop par-allel to each other. Some of the cracks have filled with water from the adjacent waterway.As the ground moves laterally, the blocks of soil between the main cracks tend to settle andbreak up into even smaller pieces.

Large-scale lateral spreads can damage all types of structures built on top of the lateralspreading soil. Lateral spreads can pull apart foundations of buildings built in the failurearea, they can sever sewer pipelines and other utilities in the failure mass, and they cancause compression or buckling of structures, such as bridges, founded at the toe of the fail-ure mass. Figure 3.42 shows lateral spreading caused by liquefaction during the PrinceWilliam Sound earthquake in Alaska on March 27, 1964, that has damaged a paved park-ing area.

Lateral spreading is discussed further in Sec. 9.5.

3.5 SLOPE MOVEMENT

3.5.1 Types of Earthquake-Induced Slope Movement

Another secondary effect of earthquakes is slope movement. As indicated in Tables 3.1 and3.2, there can be many different types of earthquake-induced slope movement. For rockslopes (Table 3.1), the earthquake-induced slope movement is often divided into falls andslides. Falls are distinguished by the relatively free-falling nature of the rock or rocks,

COMMON EARTHQUAKE EFFECTS 3.33

FIGURE 3.41 Lateral spreading caused by the Loma Prieta, California, earthquake on October 17, 1989.(Photograph from the Loma Prieta Collection, EERC, University of California, Berkeley.)


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where the earthquake-induced ground shaking causes the rocks to detach themselves froma cliff, steep slope, cave, arch, or tunnel (Stokes and Varnes 1955). Slides are different fromfalls in that there is shear displacement along a distinct failure (or slip) surface.

For soil slopes, there can also be earthquake-induced falls and slides (Table 3.2). Inaddition, the slope can be subjected to a flow slide or lateral spreading, as discussed in Secs.3.4.4 and 3.4.5.

The minimum slope angle listed in column 4 of Tables 3.1 and 3.2 refers to the mini-mum slope inclination that is usually required to initiate a specific type of earthquake-induced slope movement. Note that for an earthquake-induced rock fall, the slopeinclination typically must be 40 or greater, while for liquefaction-induced lateral spread-ing (Sec. 3.4.5) the earthquake-induced movement can occur on essentially a flat surface(i.e., minimum angle of inclination is 0.3).

3.5.2 Examples of Earthquake-Induced Slope Movement

Three examples of deadly earthquake-induced slope movements are described below.

December 16, 1920, Haiyuan Earthquake in Northern China (Mw

= 8.7). This earth-quake triggered hundreds of slope failures and landslides that killed more than 100,000people and affected an area of more than 4000 km2 (1500 mi2) (Close and McCormick1922). The landslides blocked roads and buried farmlands and villages. In one area that hada hilly topography with layers of loess ranging from 20 to 50 m (65 to 160 ft) in thickness,there were about 650 loess landslides (Zhang and Lanmin 1995).

3.34 CHAPTER THREE

FIGURE 3.42 Lateral spreading caused by the Prince William Sound earthquake in Alaska on March 27,

1964. (Photograph from the Steinbrugge Collection, EERC, University of California, Berkeley.)


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TABLE 3.1 Types of Earthquake-Induced Slope Movement in Rock

Main type of slope movement Subdivisions Material type

Falls Rockfalls Rocks weakly cemented, intensely fractured, or weathered; contain conspicuous planes of weaknedipping out of slope or contain boulders in a weakmatrix.

Slides Rock slides Rocks weakly cemented, intensely fractured, or weathered; contain conspicuous planes of weaknedipping out of slope or contain boulders in a weak

matrix.

Rock Rocks intensely fractured and exhibiting one of thavalanches following properties: significant weathering, plan

of weakness dipping out of slope, weak cementation, or evidence of previous landsliding.

Rock slumps Intensely fractured rocks, preexisting rock slump deposits, shale, and other rocks containing layers weakly cemented or intensely weathered material

Rock block Rocks having conspicuous bedding planes or slides similar planes of weakness dipping out of slopes.

Sources: Keefer (1984) and Division of Mines and Geology (1997).


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TABLE 3.2 Types of Earthquake-Induced Slope Movement in Soil

Main type of Minimum slope

slope movement Subdivisions Material type inclination

Falls Soil falls Granular soils that are slightly cemented or contain 40 Part

clay binder. (1.2 : 1) faces

Slides Soil Loose, unsaturated sands. 25 Occa

avalanches (2.1 : 1) avala

Disrupted soil Loose, unsaturated sands. 15 Ofte

slides (3.7 : 1) grou

Soil slumps Loose, partly to completely saturated sand or silt; Part

uncompacted or poorly compacted artificial fill 10 soft,

composed of sand, silt, or clay, preexisting soil (5.7 : 1) cut-a

slump deposits. plain

Soil block Loose, partly to completely saturated sand or silt; Part

slides uncompacted or slightly compacted artificial fill 5 land

composed of sand or silt, bluffs containing (11 : 1) and horizontal or subhorizontal layers of loose, foun

saturated sand or silt.

Slow earth Stiff, partly to completely saturated clay, and 10 An e

flows preexisting earth flow deposits. (5.7 : 1)

Flow slides Saturated, uncompacted or slightly compacted Inclu

Flow slides and artificial fill composed of sand or sandy silt 2.3 hollo

lateral spreading (including hydraulic fill earth dams and tailings (25 : 1) hills

dams); loose, saturated granular soils. hour

the s

Subaqueous Loose, saturated granular soils. 0.5 (110 : 1) Part

flows

Lateral Loose, partly or completely saturated silt or sand, 0.3 Parti

spreading uncompacted or slightly compacted artificial (190 : 1) floo

fill composed of sand. foun

alluv

Sources: Keefer (1984) and Division of Mines and Geology (1997).


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Loess is a deposit of wind-blown silt that commonly has calcareous cement which bindsthe soil particles together (Terzaghi and Peck 1967). Usually the loess is only weakly cementedwhich makes it susceptible to cracking and to brittle slope failure during earthquakes.

May 31, 1970, Peru Earthquake (Mw

= 7.9). This earthquake occurred offshore of cen-tral Peru and triggered a large rock slide in the Andes. The mountains are composed ofgranitic rocks, and most of the initial rock slide consisted chiefly of such rocks. The moun-tains were heavily glaciated and oversteepened by glacial undercutting.

The earthquake-induced rock slide mass accelerated rapidly as it fell over glacial icebelow the failure zone, and the resultant debris avalanche quickly became a mix of pulver-ized granitic rocks, ice, and mud (Plafker et al. 1971, Cluff 1971). The debris avalanchedestroyed all the property in its path. For example, Figs. 3.43 and 3.44 show the conditionof the city of Yungay before and after the debris avalanche. These two photographs showthe following:

The photograph in Fig. 3.43 was taken before the earthquake-induced debris avalanchewith the photographer standing in the Plaza de Armas in the central part of Yungay. Notethe palm tree on the left and the large white wall of the cathedral just behind the palm tree.The earthquake-induced debris avalanche originated from the mountains, which are visiblein the background.

In Fig. 3.44 this view is almost the same as Fig. 3.43. The debris avalanche triggered bythe Peru earthquake on May 31, 1970, caused the devastation. The same palm tree is visi-ble in both figures. The cross marks the location of the former cathedral. The massivecathedral partially diverted the debris avalanche and protected the palm trees. More than

15,000 people lost their lives in the city of Yungay.


FIGURE 3.43 This photograph was taken before the earthquake-induced debris avalanche with the pho-tographer standing in the Plaza de Armas in the central part of Yungay. Note the palm tree on the left and the

large white wall of the cathedral just behind the palm tree. Compare this figure with Fig. 3.44. (Photographfrom the Steinbrugge Collection, EERC, University of California, Berkeley.)


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March 27, 1964, Prince William Sound Earthquake in Alaska. As indicated in Sec.2.4.5, this earthquake was the largest earthquake in North America and the second-largestin this past century (the largest occurred in Chile in 1960). Some details concerning this

earthquake are as follows (Pflaker 1972, Christensen 2000, Sokolowski 2000):G The epicenter was in the northern Prince William Sound about 75 mi (120 km) east of

Anchorage and about 55 mi (90 km) west of Valdez. The local magnitude ML for thisearthquake is estimated to be from 8.4 to 8.6. The moment magnitude M

wis reported

as 9.2.

G The depth of the main shock was approximately 15 mi (25 km).

G The duration of shaking as reported in the Anchorage area lasted about 4 to 5 min.

G In terms of plate tectonics, the northwestward motion of the Pacific plate at about 2 to 3

in (5 to 7 cm) per year causes the crust of southern Alaska to be compressed and warped,with some areas along the coast being depressed and other areas inland being uplifted.After periods of tens to hundreds of years, the sudden southeastward motion of portionsof coastal Alaska relieves this compression as they move back over the subductingPacific plate.

G There was both uplifting and regional subsidence. For example, some areas east ofKodiak were raised about 30 ft (9 m), and areas near Portage experienced regional sub-sidence of about 8 ft (2.4 m).

G The maximum intensity per the modified Mercalli intensity scale was XI.

3.38 CHAPTER THREE

FIGURE 3.44 View almost the same as in Fig. 3.43. A debris avalanche triggered by the Peru earthquake

on May 31, 1970, caused the devastation. The same palm tree is visible in both figures. The cross marks thelocation of the former cathedral. The massive cathedral partially diverted the avalanche and protected thepalm trees. (Photograph from the Steinbrugge Collection, EERC, University of California, Berkeley.)


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G There were 115 deaths in Alaska and about $300 to $400 million in damages (1964 dol-lars). The death toll was extremely small for a quake of this size, due to low populationdensity, time of day (holiday), and type of material used to construct many buildings(wood).

During the strong ground shaking from this earthquake, seams of loose saturated sandsand sensitive clays suffered a loss of shear strength. This caused entire slopes to move lat-erally along these weakened seams of soil. These types of landslides devastated theTurnagain Heights residential development and many downtown areas in Anchorage. It hasbeen estimated that 56 percent of the total cost of damage was caused by earthquake-induced landslides (Shannon and Wilson, Inc. 1964, Hansen 1965, Youd 1978, Wilson andKeefer 1985).

Three examples of earthquake-induced landslides and slope movement during thisearthquake are as follows:

1. Turnagain Heights landslide: An aerial view of this earthquake-induced landslideis shown in Fig. 3.45. The cross sections shown in Fig. 3.46 illustrate the sequence of move-ment of this landslide during the earthquake. The landslide movement has been describedas follows (Nelson 2000):

During the Good Friday earthquake on March 27, 1964, a suburb of Anchorage, Alaska,known as Turnagain Heights broke into a series of slump blocks that slid toward the ocean.This area was built on sands and gravels overlying marine clay. The upper clay layers were rel-

atively stiff, but the lower layers consisted of sensitive clay. The slide moved about 610 m(2000 ft) toward the ocean, breaking up into a series of blocks. It began at the sea cliffs on the


FIGURE 3.45 Aerial view of the Turnagain Heights landslide caused by the Prince William Sound earth-quake in Alaska on March 27, 1964. (Photograph from the Steinbrugge Collection, EERC, University of

California, Berkeley.)


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ocean after about 1.5 minutes of shaking caused by the earthquake, when the lower clay layerbecame liquefied. As the slide moved into the ocean, clays were extruded from the toe of theslide. The blocks rotating near the front of the slide eventually sealed off the sensitive clay layerpreventing further extrusion. This led to pull-apart basins being formed near the rear of the slideand the oozing upward of the sensitive clays into the space created by the extension [see Fig.3.46]. The movement of the mass of material toward the ocean destroyed 75 homes on the topof the slide.

As mentioned above, the large lateral movement of this earthquake-induced landslidegenerated numerous slump blocks and pull-apart basins that destroyed about 75 homeslocated on top of the slide. Examples are shown in Figs. 3.47 to 3.51.

2. Government Hill landslide: The Government Hill School, located in Anchorage,Alaska, was severely damaged by earthquake-induced landslide movement. The schoolstraddled the head of the landslide. When the landslide moved, it caused both lateral andvertical displacement of the school, as shown in Figs. 3.52 and 3.53.

3. Embankment failure: In addition to the movement of massive landslides, such asthe Turnagain Heights landslide and the Government Hill landslide, there were smaller

3.40 CHAPTER THREE

FIGURE 3.46 The above cross sections illustrate the sequence of movement ofthe Turnagain Heights landslide during the Prince William Sound earthquake inAlaska on March 27, 1964. (Reproduced from Nelson 2000, based on work byAbbott 1996, with original version by USGS.)


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FIGURE 3.47 Damage caused by movement of the Turnagain Heights landslide during the Prince William

Sound earthquake in Alaska on March 27, 1964. (Photograph from the Godden Collection, EERC, Universityof California, Berkeley.)

FIGURE 3.48 Damage caused by movement of the Turnagain Heights landslide during the Prince WilliamSound earthquake in Alaska on March 27, 1964. (Photograph from the Steinbrugge Collection, EERC,University of California, Berkeley.)


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3.42 CHAPTER THREE


FIGURE 3.50 Damage caused by movement of the Turnagain Heights landslide during the Prince William

Sound earthquake in Alaska on March 27, 1964. (Photograph from the Steinbrugge Collection, EERC,University of California, Berkeley.)


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FIGURE 3.52 Overview of damage to the Government Hill School located at the head of a landslide caused

by the Prince William Sound earthquake in Alaska on March 27, 1964. (Photograph from the SteinbruggeCollection, EERC, University of California, Berkeley.)


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slides that resulted in substantial damage. For example, Fig. 3.54 shows earthquake-induced lateral deformation of the AnchoragePortage highway. The relatively small high-way embankment was reportedly constructed on a silt deposit (Seed 1970). Both sides ofthe embankment moved laterally, which resulted in the highway being pulled apart at itscenterline.

3.5.3 Seismic Evaluation of Slope Stability

For the seismic evaluation of slope stability, the analysis can be grouped into two generalcategories, as follows:

1. Inertia slope stability analysis: The inertia slope stability analysis is preferred for

those materials that retain their shear strength during the earthquake. There are manydifferent types of inertia slope stability analyses, and two of the most commonly usedare the pseudostatic approach and the Newmark method (1965). These two methods aredescribed in Secs. 9.2 and 9.3.

2. Weakening slope stability analysis: The weakening slope stability analysis is pre-ferred for those materials that will experience a significant reduction in shear strengthduring the earthquake. An example of a weakening landslide is the Turnagain Heightslandslide as described in the previous section.

There are two cases of weakening slope stability analyses involving the liquefaction

of soil:

3.44 CHAPTER THREE

FIGURE 3.53 Close-up view of damage to the Government Hill School located at the head of a landslidecaused by the Prince William Sound earthquake in Alaska on March 27, 1964. (Photograph from theSteinbrugge Collection, EERC, University of California, Berkeley.)


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a. Flow slide: As discussed in Sec. 3.4.4, flow failures develop when the static drivingforces exceed the shear strength of the soil along the slip surface, and thus the factor ofsafety is less than 1.0. Figures 3.38 to 3.40 show the flow slide of the Lower San

Fernando Dam caused by the San Fernando earthquake on February 9, 1971.b. Lateral spreading: As discussed in Sec. 3.4.5, there could be localized or large-

scale lateral spreading of retaining walls and slopes. Examples of large-scale lateralspreading are shown in Figs. 3.41 and 3.42. The concept of cyclic mobility is used todescribe large-scale lateral spreading of slopes. In this case, the static driving forcesdo not exceed the shear strength of the soil along the slip surface, and thus the groundis not subjected to a flow slide. Instead, the driving forces only exceed the resistingforces during those portions of the earthquake that impart net inertial forces in thedownslope direction. Each cycle of net inertial forces in the downslope direction

causes the driving forces to exceed the resisting forces along the slip surface,


FIGURE 3.54 Cracking of the Anchorage-Portage highway. Thesmall highway embankment experienced lateral movement during thePrince William Sound earthquake in Alaska on March 27, 1964.(Photograph from the Steinbrugge Collection, EERC, University ofCalifornia, Berkeley.)


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resulting in progressive and incremental lateral movement. Often the lateral movementand ground surface cracks first develop at the unconfined toe, and then the slope move-ment and ground cracks progressively move upslope.

The seismic evaluation for weakening slope stability is discussed further in Secs. 9.4

to 9.6.

3.6 TSUNAMI AND SEICHE

The final secondary effects that are discussed in this chapter are tsunamis and seiches.

Tsunami. Tsunami is a Japanese word that, when translated into English, means harbor

wave. A tsunami is an ocean wave that is created by a disturbance that vertically displacesa column of seawater. Many different types of disturbances can generate a tsunami, such asoceanic meteorite impact, submarine landslide, volcanic island eruption, or earthquake.Specifics concerning earthquake-induced tsunamis are as follows (USGS 2000a):

1. Generation of a tsunami: Tsunamis can be generated during the earthquake if theseafloor abruptly deforms and vertically displaces the overlying water. When large areas ofthe seafloor are uplifted or subside, a tsunami can be created. Earthquakes generated atseafloor subduction zones are particularly effective in generating tsunamis. Waves are

formed as the displaced water mass, which acts under the influence of gravity, attempts toregain its equilibrium.

2. Characteristics of a tsunami: A tsunami is different from a normal ocean wave inthat it has a long period and wavelength. While typical wind-generated waves may have awavelength of 150 m (500 ft) and a period of about 10 s, a tsunami can have a wavelengthin excess of 100 km (60 mi) and a period on the order of 1 h. In the Pacific Ocean, wherethe typical water depth is about 4000 m (13,000 ft), a tsunami travels at about 200 m/s (650ft/s). Because the rate at which a wave loses its energy is inversely related to its wavelength,tsunamis not only propagate at high speeds, but also travel long transoceanic distances with

limited energy losses.3. Coastal effect on the tsunami: The tsunami is transformed as it leaves the deep water

of the ocean and travels into the shallower water near the coast. The tsunamis speed dimin-ishes as it travels into the shallower coastal water and its height grows. While the tsunami maybe imperceptible at sea, the shoaling effect near the coast causes the tsunami to grow to beseveral meters or more in height. When it finally reaches the coast, the tsunami may developinto a rapidly rising or falling tide, a series of breaking waves, or a tidal bore.

4. Tsunami run-up height: Just like any other ocean wave, a tsunami begins to loseenergy as it rushes onshore. For example, part of the wave energy is reflected offshore, and

part is dissipated through bottom friction and turbulence. Despite these losses, tsunamisstill move inland with tremendous amounts of energy. For example, tsunamis can attain arun-up height, defined as the maximum vertical height onshore above sea level, of 10 to 30m (33 to 100 ft). Figure 3.55 shows a tsunami in the process of moving inland.

5. Tsunami damage: Tsunamis have great erosional ability, and they can stripbeaches of sand and coastal vegetation. Likewise, tsunamis are capable of inundating theland well past the typical high-water level. This fast-moving water associated with theinundating tsunami can destroy houses and other coastal structures. A tsunami generatedby the Niigata earthquake in Japan on June 16, 1964, caused the damage shown in Figs.3.56 and 3.57.

3.46 CHAPTER THREE


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FIGURE 3.55 Tsunami in progress. The site is the village of Kiritoppu, near Kushiro Harbor, Hokkaido,Japan. The Tokachi-oki earthquake in Japan generated the tsunami on March 4, 1952. (Photograph from theSteinbrugge Collection, EERC, University of California, Berkeley.)

FIGURE 3.56Tsunami damage caused by the Niigata earthquake in Japan on June 16, 1964. (Photographfrom the Steinbrugge Collection, EERC, University of California, Berkeley.)


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As discussed in Sec. 2.4.5, the Chile earthquake in 1960 was the largest earthquake inthis past century (moment magnitude 9.5). According to Iida et al. (1967), the tsunamigenerated by this earthquake killed about 300 people in Chile and 61 people in Hawaii.About 22 h after the earthquake, the tsunami reached Japan and killed an additional 199people. Figure 3.58 shows an example of tsunami damage in Chile.

Seiche. An earthquake-induced seiche is very similar to a tsunami, except that it devel-ops in inland waters, such as large lakes. An example of damage caused by a seiche isshown in Fig. 3.59. The building in the water was formerly on the shore about one-quartermile up the lake. The house was apparently jarred from its foundation by the earthquakeand then washed into the lake by a seiche generated by the Hebgen Lake earthquake (mag-nitude of 7.5) in Montana on August 17, 1959. The house shown in Fig. 3.59 later driftedinto the earthquake-created harbor.

Mitigation Measures. It is usually the responsibility of the engineering geologist to eval-uate the possibility of a tsunami or seiche impacting the site. Because of the tremendousdestructive forces, options to mitigate damage are often limited. Some possibilities includethe construction of walls to deflect the surging water or the use of buildings having weaklower-floor partitions which will allow the water to flow through the building, rather thanknocking it down, such as shown in Fig. 3.58.

3.48 CHAPTER THREE

FIGURE 3.57Tsunami damage caused by the Niigata earthquake in Japan on June 16, 1964. (Photographfrom the Steinbrugge Collection, EERC, University of California, Berkeley.)


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FIGURE 3.58Tsunami damage caused by the Chile earthquake in 1960. (Photograph from theSteinbrugge Collection, EERC, University of California, Berkeley.)

FIGURE 3.59 The building in the water was formerly on the shore about one-quarter mile up the lake. Thehouse was apparently jarred from its foundation by the earthquake and then washed into the lake by the seiche

generated during the Hebgen Lake earthquake in Montana on August 17, 1959. The house later drifted intothe earthquake-created harbor shown in the above photograph. (Photograph from the Steinbrugge Collection,EERC, University of California, Berkeley.)

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